Method and device for excitation and selective dissociation by absorption of laser light and application to isotopic enrichment

ABSTRACT

In a method of excitation and selective dissociation by absorption of the monochromatic light emitted by a high-power laser, the laser light is sent at a frequency ν 1  into an isotopic mixture of gaseous molecules in which a certain number of molecules exhibit transitions between two vibrational levels corresponding to a predetermined isotope and separated by an energy interval ΔE 1  = 2hν 1 . The molecules of a predetermined isotopic species are thus excited preferentially until subsequent dissociation into a number of constituents.

The present invention relates to a method of excitation and of selective dissociation by absorption of the monochromatic light emitted by a high-power laser and to a device for carrying out said method which are applicable in particular to the isotopic enrichment of uranium.

The monochromaticity of the light emitted by a laser as well as the high intensity of laser radiations obtained especially with high-power carbon dioxide lasers are such that the selective absorption of light by elements in the gaseous state is a particularly advantageous phenomenon which is adapted to the separation of isotopes. Thus it comes about as a result of the differences in mass between two isotopes that, when these isotopes are constituents of chemical substances, the levels of electronic or molecular excitation of these substances are slightly different. By adjusting the energy of the photons emitted by the laser so as to ensure that it corresponds to a transition between two energy levels associated with a substance which includes a given isotope, the molecules or the atoms containing said isotope are excited preferentially. The substances (atoms or molecules) which are thus excited preferentially and selectively are separated from the remainder of the gaseous mass by various means which make use of radiation pressure, mass spectrography, chemical reactions and so forth. It has thus been possible to carry out a large number of isotope separations on substances having an absorption spectrum such that the energy difference between two levels corresponds to the energy of the photons of the laser light.

It is readily apparent that, in order to ensure economic performance of isotope separation by laser action when utilizing the resonance absorption phenomenon described earlier, the laser employed must have good efficiency of conversion of electrical energy to light energy and its cost price must be of a low order. The CO₂ laser and especially the CO₂ laser with electronic excitation by transverse discharge (TEA laser) satisfies these two requirements.

Unfortunately, the substances exhibiting an absorption spectrum in which two levels are separated by the energy of a photon emitted by a laser of this type are relatively rare, especially in the case of gaseous compounds containing uranium.

Moreover, in the case of vapors of complex molecules containing uranium or a heavy atom, the absorption lines are usually wide and, even if a selective effect can take place in the vicinity of ten microns, its selectivity is too low to ensure that the single-photon selective absorption process described above can be applied under good conditions.

The present invention makes it possible to overcome these disadvantages and insufficiencies by employing a two-photon excitation and dissociation process. In accordance with the invention, laser light is sent into an isotopic mixture of gas molecules at a frequency ν₁ ; some of these molecules exhibit transitions between two energy levels corresponding to a predetermined isotope which is separated by the energy interval ΔE = 2 hν₁ and, by so doing, the molecules of a predetermined isotopic species are thus preferentially dissociated into a number of constituents.

In this mode of utilization, the laser light at the frequency ν₁ serves both to excite the two-photon transitions in the first vibration level or levels and to continue the excitation until dissociation of the molecule which includes the given isotope. In another mode of utilization, a first beam of laser light is employed at the frequency ν₁ for exciting the first two-photon resonant transition or transitions; a second laser beam is then sent at the frequency ν₂ in order to continue the excitation until dissociation of the molecule which includes the given isotope.

The method in accordance with the invention makes use of a so-called "two-photon" transition. In other words, the light-irradiation source is of sufficiently high intensity to ensure that transitions take place between energy levels of the molecular system which are separated by an interval ΔE = 2 hν₁. As will become apparent hereinafter, the probabilities of two-photon excitations vary as the square of the flux of incident light and can have a higher probability than the single-photon transitions in respect of very high laser light powers such as those produced by carbon-dioxide power lasers. Moreover, it may be noted in this case that the excitation energies which can be attained are within the range of 1800 cm⁻¹ to 2200 cm⁻¹ (corresponding to wavelengths between 4.5 and 5.5 microns). This can offer advantages over single-photon transitions in the case of uranium, for example when transitions of the molecular system about hν₁ do not exist.

In one of its alternative embodiments in which very high power CO₂ lasers are employed, the invention makes it possible to dissociate the molecules which undergo vibration under the action of photon excitation and vibrate more and more violently (the quantum numbers of the vibrational levels increase) until they dissociate. This process is selective: the molecules which include the isotope having vibrational energy levels corresponding to the energy ΔE = 2 hν₁ are dissociated preferentially to the others. Since the intervals between the different vibrational levels are not usually equal by reason of the anharmonicity of vibrations, these processes are possible only in the presence of light irradiation of extremely high intensity which permits compensation for inequalities of the intervals. In fact, narrowing of the energy differences between successive levels having high quantum numbers is apparently compensated by broadening of these levels due to the Stark effect for example, with the result that there always exists even in the vicinity of the dissociation a possible transition between levels in respect of a transition energy ΔE = 2 hν₁.

In one embodiment of the method according to the invention, the elements obtained by selective dissociation are eliminated or removed by causing them to react with a chemical species which is included in the mixture.

In the event that two levels having the same quantum number corresponding to two isotopic species are insufficiently separated as a result of broadening of the levels by Doppler effect, it is an advantage in accordance with the invention to employ a two-photon process in which the two photons arrive in the gas mixture in "top-to-tail" relation. As will be seen later, this eliminates the Doppler effect and makes it possible to achieve selective dissociation if the difference in levels between the two isotopic species is greater than the natural width of the two lines.

An alternative embodiment of the method according to the invention accordingly consists in separating the laser beam into two parts and in sending these two parts into the gas mixture in substantially parallel and opposite directions of propagation.

Further properties and advantages of the invention will become more readily apparent from the following description of exemplified embodiments which are given by way of explanation but not in any limiting sense, reference being made to the accompanying drawings, wherein:

FIG. 1 shows two vibrational spectra corresponding to two isotopic species excited by photons whose energy is one-half the energy interval between two vibrational levels;

FIG. 2 is an illustration of an alternative embodiment of the method according to the invention comprising a non-selective irradiation step following the selective irradiation step;

FIG. 3 shows the line spectra corresponding to two overlapping isotopic species;

FIG. 4 is a diagram of a device for carrying out the method according to the invention.

There is shown on the left-hand side of FIG. 1 a first series of vibrational levels designated as references by the quantum numbers 1, 2, 3 and 4 of said levels (in this description, the numeral 1 designates the ground-state level) corresponding to a given isotopic species. The right-hand side of the figure shows the vibration-rotation levels having the same quantum number followed by a prime index corresponding to another isotopic species of the same element; the difference ΔE' between these two vibrational levels is not the same as the difference ΔE₁ in the case of the first isotopic species. The method according to the invention consists in sending two photons having a frequency ν₁ with a combined energy 2 hν₁ which excite the transition from level 1 to level 2, from level 2 to level 3 and so forth until dissociation of the molecule occurs, the level ΔE₁ being equal to twice the energy hν₁ of each photon. The rotational levels corresponding to each vibration are shown at 2a, 2b and so on. The process of excitation and dissociation is selective if the difference ΔE₁ - ΔE' is greater than the width of a vibration-rotation line.

FIG. 2 gives an explanatory diagram of an alternative mode of execution of the method according to the invention in which a level 2 is selectively excited by two photons having a frequency ν₁ followed by a second irradiation having an energy hν₂ which causes the molecules or atoms of state 2 to pass to state i in which the molecule is dissociated. The irradiation having the energy ΔE₂ is not selective but finally results in selective dissociation since only the level 2 is populated by the two-photon selective irradiation.

FIG. 3 shows the profiles of the lines corresponding for example to two vibrational levels having the same quantum number of two isotopic species.

In FIG. 3, the frequency has been plotted as abscissae and the intensity I of the two lines corresponding for example to the level 2 and 2' of FIG. 1 has been plotted as ordinates. The curves 6 and 8 correspond to the profiles of the two lines in respect of two isotopes of mass m₁ and m₂. It is apparent that these lines which are broadened by Doppler effect overlap to too great an extent to be readily separated by any two-photon process having a selectivity which would be too low. On the other hand, the curves 10 and 12 represent the natural widths of these two lines in the absence of a Doppler effect. The influence of the Doppler effect can be eliminated by the action of two (top-to-tail) photons which travel towards each other. In this case, the two waves corresponding to the two groups of photons travel in opposite directions. In the case of a molecule having the velocity v, the two waves correspond to frequencies ν₁ ± (k/2π)v, where +k corresponds to the wave number of the wave which travels in one direction and -k corresponds to the wave which travels in the opposite direction. When the two photons are absorbed simultaneously, the resonance condition is written (ν₁ + (k/2π)v) + (ν₁ - (k/2π)v) = 2ν₁ = ΔE₁ /h and the Doppler effect has disappeared.

The utilization of this phenomenon makes it possible to separate the two lines which retain only their natural width, thus enhancing the selectivity of the method to a considerable extent. Thus in the case of isotopes having closely related masses at relative value (as in the case of uranium, for example), it will be an advantage to employ two separate beams having the same density and propagating towards each other, with the result that the excitation by two photons has high selectivity.

Thus the method according to the invention makes it possible to combine with high absorptions in the vicinity of 5 microns an effect of selectivity which is increased by two beams propagating in opposite directions.

FIG. 4 shows a device for carrying out the method according to the invention: by way of example, the laser 14 is a TEA laser which provides power outputs of the order of 1 GWatt/cm². The emergent laser beam 16 is separated by a semi-reflecting plate 18 of germanium for example into two beams 20 and 22 of substantially equal intensity. Said beam is reflected from the mirrors 24, 26 and 28 towards the enclosure 30 which is filled with the gaseous isotopic mixture such as a mixture of uranium hexafluoride and hydrogen, for example. The energy of the two beams is concentrated within the enclosure 30 by means of the lenses 32 and 34 of germanium, barium fluoride or sodium chloride, for example. The power level of the vicinity of the point of convergence 36 of the two lenses is sufficient to ensure that the two-photon excitation processes are appreciable.

In order to adapt the energy of a photon to one-half the energy corresponding to a vibrational transition, it will be possible to employ different isotopes of carbon and oxygen in the laser mixture.

Among the absorbent molecules in the vicinity of 5 microns, mention can be made of nitrosyl which absorbs in the NO absorption band in the vicinity of 1888 cm⁻¹, nydopenta-borane which absorbs at 1840 cm⁻¹, carbon monoxide which absorbs in the vicinity of 2140 cm⁻¹ whose energy is double the value corresponding to the line P(18) of a C¹⁸ O₂ laser (1070.60 cm⁻¹).

In the different examples given below, the molecules which absorb at double the frequency of the CO₂ laser are dissociated then separated chemically:

EXAMPLE 1

Irradiation of nitrosyl: the nytrosyl irradiated by photons derived from a CO₂ laser gives rise to the following reactions:

    NO → N + O

    o + h.sub.2 → h.sub.2 o

    n + 3/2 h.sub.2 → nh.sub.3

the products H₂ O and NH₃ are enriched in the isotope O¹⁷, O¹⁸ or N¹⁵ corresponding to the selected line in the NO absorption band located in the vicinity of 1888 cm⁺¹. The difference between the lines is sufficient to obtain acceptable selectivity and it is not necessary to employ two beams which propagate in opposite directions.

EXAMPLE 2

The isotopes B¹¹ and B¹⁰ of boron can be dissociated by irradiation of nydopentaborane, the boron of the selective isotopic species being trapped by action of bromine in accordance with the reactions:

    B.sub.5 H.sub.9 → 5 B + 9 H

B +3/2 br₂ → B Br₃

it being possible to utilize the vibrational frequency of pentaborane of 1840 cm⁻¹ at the time of a process involving two photons derived from a CO₂ laser.

EXAMPLE 3

The isotopes of carbon and of oxygen can be separated by irradiation of a mixture of carbon monoxide and chlorine at a frequency in the vicinity of 2140 cm⁻¹. In this case, the product of the reaction, namely phosdene COCL₂ is enriched in the carbon isotope corresponding to the irradiated isotopic species. In these first three examples, the vibrational levels are sufficiently distinct by virtue of the low mass of the isotopic compounds to avoid the need for irradiation of the "top-to-tail" type with two beams propagating in opposite directions.

EXAMPLE 4

The use of two photons which propagate in opposite directions makes it possible to apply the method according to the invention with highly enhanced selectivity to molecules which do not have well-defined absorption lines among the different isotopic species of interest. In the case of uranium hexafluoride UF₆, the levels 3ν₃ (1870 cm⁻¹) and 2ν₁ + ν₂ (1862 cm⁻¹) can both be excited by two-photon transitions having a wavelength in the vicinity of 10.6 microns; the second level is accessible from the ground state by means of a two-photon electrical dipole transition. In this case the number of transitions per second W between two levels in respect of a light power P is equal to W = δ₂ ρ², the power being expressed in photons cm⁻² sec ⁻¹. The section δ₂ is of the order of 10⁻⁵⁰ cm/sec and the effective cross-section of a transition to a photon is typically of the order of 10⁻²⁰ cm². A light power P of the order of 1.35 × 10²⁸ photons cm⁻² sec, namely approximately 250 MW cm⁻² in respect of the typical emission wavelength of a CO₂ laser, results in the fact that the two-photon excitation is in that case just as efficient as a one-photon excitation at the power threshold corresponding to dissociation of the molecule. This threshold value is of the order of a few tens of Mwatts per cm² and can be attained by means of two-photon transitions. The width of the line emitted by the laser must be at least of the order of the frequency difference between the vibration-rotation lines of the two isotopic species, namely of the order of 50 MHertz.

In order to carry out this separation, it is possible to employ the device shown in FIG. 4; the pressure within the enclosure 30 is sufficiently low, namely of the order of 1 torr in order to prevent de-activating and non-selective collisions. The mixture of uranium hexafluoride and of one molecule which is capable of reacting on the dissociation products such as hydrogen for example is introduced into the enclosure. The beam emitted by the laser 14 has a power of 1 GWatt cm⁻². The laser emits pulses having a duration of 100 nanoseconds which give rise to this selective dissociation of uranium hexafluoride and to the following chemical reactions:

    .sup.235 UF.sub.6 → .sup.235 UF.sub.4 + 2 F

    h.sub.2 + 2f → 2 hf

the depleted uranium hexafluoride is then discharged: the repetition frequency of the irradiation pulses can attain several tens of Hertz, thus ensuring a suitable flow rate of uranium hexafluoride enriched in uranium-235. 

I claim:
 1. A method of excitation and of selective dissociation by absorption of the monochromatic light emitted by at least one high-power laser, wherein the laser light is sent at a frequency ν₁ into an isotopic mixture of gas molecules in which a certain number of said molecules exhibit transitions between two vibrational levels corresponding to a predetermined isotope and separated by an energy interval ΔE₁ = 2hν₁, the molecules of a predetermined isotopic species are thus preferentially excited until subsequent dissociation into a number of constituents, and a gas is added to the mixture which added gas reacts with the constituents derived from the dissociated molecules of a predetermined isotopic species.
 2. An application of the method according to claim 1 to the enrichment of uranium, wherein the isotopic mixture comprises molecules A containing uranium and molecules B which react with the products of dissociation of the molecules A.
 3. An application according to claim 2, wherein the molecules A are molecules of an isotopic mixture of uranium hexafluoride.
 4. An application according to claim 3, wherein the molecules B are hydrogen molecules.
 5. A method for the enrichment of uranium by at least one high power laser, wherein the laser light is sent into an isotopic mixture comprising molecules A containing uranium and molecules B which react with the products of dissociation of the molecules A.
 6. The method according to claim 5, wherein molecules A are molecules of an isotopic mixture of uranium hexafluoride.
 7. The method according to claim 6, wherein the molecules B are hydrogen molecules. 